1. Field of the Invention
The present invention relates to a spectroscopic ellipsometer and a method of focusing in the spectroscopic ellipsometer, and also relates to a film thickness measuring apparatus having the spectroscopic ellipsometer.
2. Description of the Background Art
Conventionally, an ellipsometer is used as an optical measuring apparatus for measuring a thickness or a surface state of a film formed on an object. In the ellipsometer, polarized light is applied to a measurement surface of the object so as to incline to the measurement surface, a polarization state of reflected light reflected on the measurement surface is acquired and ellipsometry is performed to measure a thickness of a film formed on a substrate and a surface state of the measurement surface. For example, U.S. Pat. No. 5,608,526 (Document 1) and Japanese Patent Application Laid-Open No. 2005-3666 (Document 2) disclose a spectroscopic ellipsometer for performing various measurements (e.g., film thickness measurement) on a thin film formed on an object on the basis of a polarization state at each wavelength of reflected light.
In such a spectroscopic ellipsometer, focusing is performed by moving an object in a vertical direction so that a convergence position where polarized light applied to the object is most converged is located on a measurement surface of the object. If the measurement surface of the object slightly moves from a position in a vertical direction where the measurement surface of the object overlaps with the convergence position (the position is referred to as a “focus position”), a position of an irradiation region on the measurement surface of the polarized light also moves. For this reason, in the spectroscopic ellipsometer, it is necessary that focusing is accurately performed before ellipsometry and especially, in a case where ellipsometry of a semiconductor substrate in which a fine pattern is formed or the like is performed, focusing with higher accuracy should be performed.
However, normally, the focal depth of light which enters an object so as to be perpendicular to the object is deeper than that of light which enters a measurement surface of an object so as to incline to the measurement surface. If focusing is performed with an observation optical system of vertical incident light type like in Document 1, there is a case where a required accuracy in focusing is not satisfied in a measurement optical system for ellipsometry where polarized light is applied to the object so as to incline to the object.
In order to achieve high accurate focusing, a relative position of the two optical systems must be adjusted so that an irradiation region by the measurement optical system for ellipsometry overlaps with an irradiation region by the observation optical system for focusing with accuracy. In the spectroscopic ellipsometer of Document 1, however, since the two optical systems are provided independently of each other, there is a possibility that the relative position of the both optical systems is apart from the original position in adjusting, because of a difference between expansion rates by temperatures of the optical systems or the like.
In the spectroscopic ellipsometer of Document 2, light emitted from an observation optical system for focusing is incident on the object so as to incline to the object like in that emitted from a measurement optical system for ellipsometry, and it is therefore possible to prevent decrease of the accuracy of focusing by a difference between focal depths. However, since a substrate imaging part and an optical system around the substrate imaging part are provided independently of the measurement optical system, it is difficult to prevent positional shift of the relative position of the two optical systems and there is a limitation in improving the accuracy of focusing.